Constraints and Errors An ounce of prevention Outline Questions? Project reminder Recap Norman About errors Mistakes and slips Error guidelines Project reminder Brainstorm now! Narrow down to 3 to 4 different ideas Vary features, input/out, platform, usability Do low fidelity sketches/storyboards/scenarios/etc. Storyboard assignment: due next Wednesday Poster: in class on Oct. 20 Recall: Norman Design Concepts Affordances Visibility Conceptual models Mapping Feedback Constraints Affordances Whats wrong with this picture? Constraints Limitations on what can be done Physical - keys Semantic - menu graying Cultural - Colors Logical - When all above dont apply Simple Example What if both sides were big and you had to remember which side the small one went into? Electric plugs Knowledge in Head vs. Knowledge in the World Head Not easy to retrieve Learning required, good conceptual model makes easier Can be very efficient Not easy first time Aesthetics do not need to make info visible World Easy to retrieve No learning, only interpretation Use slowed by need to find the info to interpret Easy for first time Can be cluttered or inelegant Execution-Evaluation cycle Norman (DOET, p. 46) User Goals Physical System Gulf of Execution Gulf of Evaluation Goals, Execution, Evaluation Physical System Goals What we want to happen Execution What we do to the world Evaluation Comparing what happened with what we wanted to happen (Gulf of Execution) (Gulf of Evaluation) Execution Goals What we want to happen An intention to act so as to achieve the goal The actual sequence of actions that we plan to do The physical execution of that action sequence Physical System Evaluation Goals What we want to happen Evaluation of the interpretations with what we expected to happen Interpreting the perception according to our expectations Perceiving the state of the world Physical System Seven Stages - All Together Goals What we want to happen Evaluation of the interpretations with what we expected to happen Interpreting the perception according to our expectations Perceiving the state of the world An intention to act so as to achieve the goal The actual sequence of actions that we plan to do The physical execution of that action sequence Physical System Implications Which Gulf does these Address? Make current state and action alternatives visible Need good conceptual model with consistent system image Interface should include mappings that reveal relationships between stages User should receive continuous feedback Provide affordances Goal: Minimize Gulfs Gulf of Execution Conceptual model Affordances Natural mappings Gulf of Evaluation Make state visible Feedback Errors - Three Considerations Avoiding and preventing Identifying and understanding Handling and recovering Why errors are important Errors are unavoidable To err is human Making mistakes is part of learning Designers responsibility Understand why errors occur Minimize likelihood Allow for recognition of error and graceful recovery (forward or backward) Example Studies Errors Happen! 170 experienced UNIX users over 9 days Individual commands had error rates of 3-50% 300 security system users over 20 months 12,117 error messages Most common 11 errors -> 65% 2517 involved repeated errors (with no non-errors in between) within 10 minutes Bad error recovery/help Kraut et al, CHI 83 Mosteller & Ballas, Human Factors 89 Perceptual Errors Result from insufficient or poor perceptual cues Display of objects that are visually similar Invisible or poorly expressed states Failure to capture users attention Lack of perceivable feedback Cognitive Errors Caused by taxing the memory and problem solving capabilities Tax recall memory Lack of or poor mnemonic aids Inconsistency Lack of context or status info e.g., where came from in a menu Mental calculations and translations Motor Errors Taxing the eye-hand coordination and motor skills Awkward motor movements Highly similar motor sequences e.g., double click, click Pressure for speed Require a high degree of hand-eye coordination Requiring special types of motor skills (type) Mistakes and Slips Mistakes (hopefully avoidable) Wrong intention Incorrect mental model Novice behavior Slips (unavoidable) Wrong execution Skilled behavior Moral slips happen Types of Slips 1. Capture error - Continue frequently done activity instead of intended one (similar starts) Confirm deletion of file instead of cancel 2. Description error - Intended action has much in common with others possible (usually when distracted, close proximity) shift key & caps lock key Types of Slips 3. Data driven error - Triggered by arrival of sensory info which intrudes into normal action Call to give someone a number, dial that number instead 4. Associative activation - Internal thoughts and associations trigger action Phone rings, yell come in Types of Slips 5. Loss of activation - Forgetting goal in middle of sequence of actions Start going into room, then forget why youre going there 6. Mode errors - Do action in one mode thinking youre in another Delete file, but youre in wrong directory Scroll wheel in PowerPoint What to do? As much as possible Prevent mistakes Allow recovery from slips Error Prevention Guidelines Eliminate modes or provide visible cues for modes - no invisible modes Use consistent representation (color, style) Maximize recognition, minimize recall Design non-similar motor sequences for commands Minimize need for typing Test and monitor for errors and engineer them out Allow reconsideration of action by user (e.g., removing file from trash) Avoid ambiguous and unclear prompts and messages - as in next set of examples! Great Examples Error Recovery Guidelines Provide appropriate type of response Gag - Prevent user from continuing Erroneous login Warn - Warn user an unusual situation is occurring Bell or alert box Nothing - Dont do anything (Careful, user must determine problem) move file to bad place Self-correct - Guess correct action & do it Spell-check correction Dialog - System opens dialog with user Go into debugger on run-time crash Error Recovery Guidelines Provide undo function Provide cancel function from operations in progress Require confirmation for drastic, destructive commands Provide reasonableness checks on input data Did you really mean to order 5000? Return cursor to error field, allow fix Provide some intelligence Guess what they wanted to do Provide quick access to context-sensitive help Error Message - What to Say Error:Error code -37 Description:Disk full Prescription:Disk full; recover disk space Prescription + aid:Disk full; recover space by deleting files or defragmenting Prescription + offer:Disk full; proceed with disk defragmentation? Otherwise delete files. How Does This Example Fit In? Error Message Wording - Vocabulary Problem with previous example - some users will not know what defragmentation means!! Vocabulary User-oriented Defined in advance for commonality throughout all messages (in style guide) Alternatives to defragmentation ? Error Message Wording - Tone Sorry, command not recognized Command not recognized :-( Command not recognized Command not recognized!! Implications for design Scenarios should include errors !!! Dont forget this in your project! *hint hint* Distinguish between skilled and learner errors Uncover errors in the existing system how do people self-monitor (cheat sheets)